1. Field of the Invention
The present invention relates to ion sensitive field effect transistors (ISFETs), and more particularly to ion sensitive field effect transistors with a sensitive contact window for detecting the concentration of ions or electrically polarized or charged objects, such as proteins, molecules, e.t.c.
2. Description of the Related Art
The functions of Ion sensitive field effect transistor (ISFET) are based on ion adsorption onto sensing gate which can induce a Helmholtz voltage, modulating the source-drain current. The performance of ISFET is primarily determined by source-drain current/voltage sensitivity or by gate sensitivity at a constant source-drain current. The gate sensitivity means the induced Helmholtz voltage at gate regions with respect to the concentration change (theoretical Nernst constant of 59.16 mV per decade at 25° C.). The current/voltage sensitivity transduces the input gate voltage into the source-drain current. Optimization of the gate sensitivity and the current/voltage sensitivity is crucial for ISFET applications in biomedical and chemical sensing, wherein the current/voltage sensitivity and the gate sensitivity can be influenced by materials or device structures of the ISFET.
The U.S. patents related to the formation of the ISFET are listed hereinafter.
(1) U.S. Pat. No. 4,735,702 discloses a polymer coated on an oxide layer of ISFET, wherein a chemical bond is formed on the interface between the polymer and the oxide layer to form a sensitive film.
(2) U.S. Pat. No. 5,061,976 discloses a method that a carbon thin film is coated on the gate oxide of the ISFET and then a 2,6-xylenol electrolytically polymerized film is coated thereon.
(3) U.S. Pat. No. 5,314,833 discloses a method comprising steps of depositing a silicon film on a GaAs substrate and doping arsenic/phosphorous ions into the silicon film to fabricate the gate with lower resistance.
(4) U.S. Pat. No. 5,319,226 discloses a Ta2O5 sensing film deposited by a radio frequency sputtering method on an ISFET, wherein the ISFET consists of a Ta2O5/Si3N4/SiO2 structure.
(5) U.S. Pat. No. 5,387,328 discloses a method of measuring the glucose concentration by fixing the enzyme on a sensing film and using platinum (Pt) as a reference electrode. The sensor has a Pt electrode being capable of sensing all biological substances which generate H2O2 in enzyme reaction.
(6) U.S. Pat. No. 6,531,858 B2 discloses a method of measuring the hysteresis value and the drift value of an a-Si:H ISFET.
(7) U.S. Pat. No. 6,573,741 B2 a method and an apparatus for measuring the temperature parameters of an ISFET that uses hydrogenated amorphous silicon as a sensing film.
(8) U.S. Pat. No. 6,617,190 B2 discloses an ISFET comprising an H+-sensing membrane consisting of RF-sputtering a-WO3.
(9) U.S. Pat. No. 7,387,923B2 discloses an ISFET using a PbTiO3 layer as a sensing film to detect H+ ions and the fabrication of the PbTiO3 sensing film by a Sol-Gel process.
Recently, group-III nitrides, such as AlGaN, GaN, and InN, are found to exhibit high sensitivity and robust surface properties against the chemical damages. The sensors comprised of III nitride along with electronic readout are noticed to be promising for next generation sensors.
Among group-III nitrides, InN exhibits an unusual phenomenon of strong surface electron accumulation, and has the potential to realize a high current/voltage sensitivity ISFET. The carriers in InN based ISFET transport through surface, bulk and interface channels of the transistors. The carriers in the surface channel are attributed to the surface electron accumulation which is confirmed by various experimental techniques such as capacitance-voltage (C-V) measurements or high-resolution electron-energy-loss spectroscopy (HREELS), etc. The electron accumulation property corresponds to the particularly low conduction band minimum at Γ-point and the Fermi stabilization energy (EFS) deep inside the conduction band. The carriers in the interface channel, induced by the band bending of interface polarization charges, depend on the polarity and the lattice mismatch between the InN epilayer and the underlying buffer layer. The sheet carrier density at the polar InN/AlN layer interface is about one order of magnitude higher than that at the nonpolar InN/GaN layer interface due to the differences in spontaneous polarization. The carriers in the bulk channel are of n-type for as-grown undoped InN epilayer, and are probably to be of p-type for Mg-doped InN epilayer. Among the three carrier channels, the surface channel is most adjacent to the layer for ion adsorption, strongly affecting the channel current.
In the present invention, a novel ISFET with a Mg-doped InN (InN:Mg) epilayer is disclosed.